Electrolyte for obtaining melts using an aluminum electrolyzer

Abstract

The invention concerns non-ferrous metallurgy, in particular the composition of an electrolyte for electrically obtaining aluminum by the electrolysis of fluoride melts. The electrolyte proposed contains, in % by weight: sodium fluoride 26-43, potassium fluoride up to 12, lithium fluoride up to 5, calcium fluoride 2-6, alumina 2-6, aluminum fluoride and admixtures—the remainder. The technical result is to increase the solubility of alumina in the electrolyte at a temperature of 830-930° C. In the electrolyte being applied for, the carbon and inert electrode materials are not destroyed, and the use of special methods to purify the aluminum of melt components is not required.

Claims

1. An electrolyte for obtaining aluminum by the electrolysis of melts consisting of components in the following quantitative proportions, in % by weight: NaF—26, KF—12, LiF—5, CaF.sub.2—2 to 2.5, Al.sub.2O.sub.3—2 to 2.5, AlF.sub.3—remainder, wherein the electrolyte has an alumina solubility of about 5 wt % at a temperature of 830-930° C., has a liquidus temperature of about 750° C., and has a specific electrical conductivity of about 1.7 Ω.sup.−1 cm.sup.−1 at a temperature of 830° C.

2. An electrolyte for obtaining aluminum by the electrolysis of melts consisting of components in the following quantitative proportions, in % by weight: NaF—43, CaF.sub.2—5 to 6, Al.sub.2O.sub.3—5 to 6, AlF.sub.3—remainder, wherein the electrolyte has an alumina solubility of about 7.5 wt % at a temperature of 830-930° C., has a liquidus temperature of about 900° C., and has a specific electrical conductivity of about 2.0 Ω.sup.−1 cm.sup.−1 at a temperature of 930° C.

Description

(1) The task of the invention is the increase productivity and to reduce the cost price of obtaining aluminum at an electrolysis temperature of 830-930° C.

(2) The technical result is an increase in alumina solubility in the electrolyte at the temperature of 830-930° C. In the electrolyte being applied for, the carbon and inert electrode materials are not destroyed, and the use of special methods is not required to purify the aluminum of melt components.

(3) The problem posed is solved, in that the electrolyte for obtaining aluminum by the electrolysis of melts contains, according to the composition being applied for, sodium fluoride, potassium fluoride, lithium fluoride, calcium fluoride, and alumina, and aluminum fluoride and admixtures, in the following quantitative component proportions, in %.sub.wt:

(4) NaF—26-43,

(5) KF—up to 12,

(6) LiF—up to 5,

(7) CaF.sub.2—2-6,

(8) Al.sub.2O.sub.3—2-6,

(9) AlF.sub.3 and admixtures—remainder.

(10) The admixtures in the electrolyte are usually the oxide and fluoride constituents of iron, silicon, magnesium, etc. The electrolyte has a liquidus temperature of 750-900° C., and at an electrolysis temperature of 830-930° C., it has an alumina solubility of over 5%.sub.wt. During electrolysis, the electrolyte does not destroy the carbon and inert electrode materials, and the aluminum being obtained is not contaminated with melt components, particularly lithium.

(11) The essence of the proposed solution is as follows.

(12) Use of an electrolyte with a reduced liquidus temperature at an electrolysis temperature of 830-930° C. allows productivity to be increased in the process of obtaining aluminum by the electrolysis of molten salts and the cost price of obtaining aluminum to be reduced.

(13) Sodium fluoride and aluminum fluoride are the principal components of the electrolyte. At a NaF content of less than 26%.sub.wt, the electrical conductivity of solubility of alumina is significantly reduced. A content of more than 43%.sub.wt NaF does not allow the electrolysis temperature to be reduced to less than 930° C.

(14) Lowering the liquidus temperature is also achieved by introducing the additives KF, LiF, and CaF.sub.2. All these additives, other than KF, cause a reduction in the solubility of aluminum oxide in the melt. At the same time, the electrical conductivity of the electrolyte is reduced by introducing potassium fluoride, and the destruction of carbon materials is accelerated as a result of introducing potassium into their structure. Decreasing the electrolyte temperature also leads to a decrease in alumina solubility. Therefore, the lower the electrolyte temperature, the greater the content of potassium fluoride must be. However, with a content of KF greater than 12%.sub.wt, deformation and destruction of carbon anodes and cathodes occurs, due to introducing potassium into them, and the electrical conductivity of the electrolyte is substantially reduced in addition.

(15) A LiF additive is used to increase the electrical conductivity of the electrolyte and simultaneously decrease the liquidus temperature. But at a LiF content of more than 5%.sub.wt (up to 40%.sub.wt in the prototype) and an electrolyte temperature of less than 930° C., the solubility of the alumina in the melt is substantially reduced. In addition, lithium concentration in the aluminum rises with a lithium content of more than 5%.sub.wt which requires the application of special methods to purify the metal obtained. Otherwise problems arise during its processing, for example during the manufacture of aluminum foil [B. J. Welch et. al., Aluminium smelter technology: theory and practice].

(16) A content of 2-6%.sub.wt calcium fluoride cannot be avoided, due to the input of calcium into the electrolyte in the alumina composition. The additional introduction of calcium fluoride above that of background leads to a decrease in alumina solubility and the electrical conductivity of the melt.

(17) Magnesium fluoride alters the properties of the electrolyte analogously to calcium fluoride, but the background content of magnesium fluoride here is considerably lower and may therefore pertain to admixture contamination of the melt. The additional introduction of magnesium fluoride (2-8%.sub.wt in the prototype) leads, as in the case of calcium fluoride, to a decrease in alumina solubility and in electrical conductivity of the melt.

(18) With low alumina solubility in the electrolyte (not more than 4-5%.sub.wt in the prototype) and high anode current density (0.8 A/cm.sup.2 or more), the concentration of alumina dissolved in the melt can be less than 2%.sub.wt. Then the deficit in oxygen-bearing ions rises at the surface of the anodes, and the decomposition of the fluoride components of the electrolyte begins, which leads to an increase in electrolyzer voltage, anode degradation, elevated consumption of the fluorine salts, and the evolution of deleterious fluorine-bearing gases. At a concentration higher than 6% wt, the formation of alumina precipitates is possible in the electrolyzer, which disrupts the electrolysis process technologically.

(19) The existence of a cause-and-effect relationship between the aggregate essential traits of the subject being applied for and the achievable technical result is shown in the table.

(20) TABLE-US-00001 TABLE Explanation, whereby (distinguishing traits and/or an aggregate thereof) improvement was possible in the indices Type of technical of the proposed subject, result and their Actual or calculated indices compared with the dimensions Prototype Subject being applied for prototype Composition, %.sub.wt AlF.sub.3 = 30 NaF = 26 NaF = 43 A decrease in the content LiF = 10 KF = 12 CaF.sub.2 = 5 of lithium fluoride and a CaF.sub.2 = 2.5 LiF = 5 Al.sub.2O.sub.3 = 5 corresponding increase in MgF.sub.2 = 2.5 CaF.sub.2 = 2.5 AlF.sub.3 = 47 total content of sodium KF = 2.5 Al.sub.2O.sub.3 = 2.5 fluoride and potassium Al.sub.2O.sub.3 = 2.5 AlF.sub.3 = 52 fluoride, i.e. components Na.sub.3AlF.sub.6 - 50 (of increasing the solubility these, NaF = 30) of alumina in the electrolyte Liquidus temperature, 839 750 900 being applied for ° C. compared with the prototype, Operating temperature, 830 830 930 ensure an increase ° C. in the solubility of alumina Specific electrical 2.5 1.7 2.0 in the electrolyte at conductivity (at a temperature of 830-930° C. operating temperature), Ω/.sup.−1 cm.sup.−1 Solubility of 2.3 5.2 7.5 alumina (at operating temperature), % wt

(21) Thus, the invention allows, at a temperature of 830-930° C., the solubility of alumina in the electrolyte to be increased up to 5% wt or more and, consequently, allows the productivity of electrolyzers operating at an electrolysis temperature of 830-930° C. to be increased. In addition, the invention allows aluminum to be obtained which is not contaminated with lithium and which does not require the application of special purification methods. Consequently, the invention allows the cost price of obtaining aluminum to be reduced.